RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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RADIATION PROTECTION INDIAGNOSTIC
ANDINTERVENTIONAL RADIOLOGY
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• L 8 Factors affecting image quality

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Introduction

• A review is made of
• Definitions of image quality parameters
• The factors that affect the image quality
• The common image quality related problems
  encountered by radiologists in routine practice
• The image criteria concept as a tool to help to
  achieve good image quality with the use of low
  radiation dose per radiograph

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Topics

• Image quality evaluators
• Image contrast
• Blur or lack of sharpness
• Distortion and Artifacts
• Image noise

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Overview

• To become familiar with the factors that
  determine the image clarity and the way the image
  quality can be improved

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Part 8 Image quality
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 1 Basic Image Quality Evaluators

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Imaging quality

• Efficient diagnosis requires
• acceptable noise
• good image contrast
• sufficient spatial resolution
• These factors are linked
• Objective measurement of quality is difficult

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Factors affecting image quality
Blur or Unsharpness
Contrast
Image quality
Distortion artifact
Noise
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Image quality evaluators/descriptors

• Basic evaluators
• Contrast
• Resolution
• Noise
• Linking evaluators
• Modulation transfer
• Signal-to-noise ratio
• Wiener spectra
• Overall evaluators
• Contrast detail analysis
• Rose Model
• ROC analysis

NOISE
Contrast detail analysis Rose model ROC
analysis
Signal-to-noise Ratio S/N
Wiener spectra
RESOLUTION
CONTRAST
Modulation transfer Function MTF
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Part 8 Image quality
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 2 Image contrast

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Image contrast
High Contrast
Low Contrast
Medium Contrast
Image contrast refers to the fractional
difference in optical density of brightness
between two regions of an image
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Some factors influencing contrast

• Radiographic or subject contrast
• Tissue thickness
• Tissue density
• Tissue electron density
• Effective atomic number Z
• X Ray energy in kV
• X Ray spectrum (Al filter)
• Scatter rejection
• Collimator
• Grid

• Image contrast
• The radiographic contrast plus
• Film characteristics
• Screen characteristics
• Windowing level of CT and DSA

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Technique factors (1)

• Peak voltage value has an influence on the beam
  hardness (beam quality)
• It has to be related to medical question
• What is the anatomical structure to investigate?
• What is the contrast level needed?
• For a thorax examination 130 - 150 kV is
  suitable to visualize the lung structure
• While only 65 kV is necessary to see bone
  structure

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Technique factors (2)

• The higher the energy, the greater the
  penetrating power of X Rays
• At very high energy levels, the difference
  between bone and soft tissue decreases and both
  become equally transparent
• Image contrast can be enhanced by choosing a
  lower kVp so that photoelectric interactions are
  increased
• Higher kVp is required when the contrast is high
  (chest)

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X Ray penetration in human tissues
60 kV - 50 mAs
70 kV - 50 mAs
80 kV - 50 mAs
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X Ray penetration in human tissues
Improvement of image contrast (lung)
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Technique factors (3)

• The mAs controls the quantity of X Rays
  (intensity or number of X Rays)
• X Ray intensity is directly proportional to the
  mAs
• Over or under-exposure can be controlled by
  adjusting the mAs
• If the film is too white, increasing the mAs
  will bring up the intensity and optical density

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X Ray penetration in human tissues
70 kV - 25 mAs
70 kV - 50 mAs
70 kV - 80 mAs
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Receptor contrast

• The film as receptor has a major role to play in
  altering the image contrast
• There are high contrast and high sensitivity
  films
• The characteristic curve of the film describes
  the intrinsic properties of the receptor (base
  fog, sensitivity, mean gradient, maximum optical
  density)
• N.B. Film processing strongly has a pronounced
  effect on fog and contrast

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Video monitor

• The video monitor is commonly used in fluoroscopy
  and digital imaging
• The display on the monitor adds flexibility in
  the choice of image contrast
• The dynamic range of the monitor is limited
  (limitation in displaying wide range of
  exposures)
• Increased flexibility in displaying image
  contrast is achieved by adjustment of the window
  level or gray levels of a digital image

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Contrast agents

• Nature has provided limited contrast in the body
• Man-made contrast agents have frequently been
  employed to achieve contrast when natural
  contrast is lacking (iodine, barium)
• The purpose is to get signals different from the
  surrounding tissues and make visible organs that
  are transparent to X Rays

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Part 8 Image quality
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 3 Blur or lack of sharpness

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Blur or lack of sharpness

• The boundaries of an organ or lesion may be very
  sharp but the image shows a lack of sharpness
• Different factors may be responsible for such a
  degree of fuzziness or blurring
• The radiologist viewing the image might express
  an opinion that the image lacks detail or
  resolution (subjective reaction of the viewer
  to the degree of sharpness present in the image)

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Resolution

• Smallest distance that two objects can be
  separated and still appear distinct
• Example of limits
• Film screen 0.01 mm
• CT .5 mm
• Other definition Point-spread function
• Characteristic of a point object
• Point object expected to be point in image
• Blurring due to imperfections of imaging system
• Measurement full-width-at-half-maximum FWHM

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Factors affecting image sharpness
Subject Unsharpness
Geometric Unsharpness
Image Unsharpness
Motion Unsharpness
Subject Unsharpness
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Geometric blur

• If the focal spot is infinitesimally small, the
  blur is minimized because of minimal geometric
  bluntness
• As the focal spot increases, the blur in the
  image increases

Small focal spot
Large focal spot
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Geometric blur

• Another cause of lack of geometric sharpness is
  the distance of the receptor from the object
• Moving the receptor away from the object results
  in an increased lack of sharpness
• N.B. The smaller the focal size and closer the
  contact between the object and the film (or
  receptor), the better the image quality as a
  result of a reduction in the geometric sharpness

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Lack of sharpness in the subject

• Not all structures in the body have well-defined
  boundaries (superimposition essentially present
  in most situations)
• The organs do not have square or rectangular
  boundaries
• The fidelity with which details in the object are
  required to be imaged is an essential requirement
  of any imaging system
• The absence of sharpness, in the subject/object
  is reflected in the image

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Lack of sharpness due to motion (1)

• Common and understandable blur in medical imaging
• Patient movement
• uncooperative child
• organ contraction or relaxation
• heart beating, breathing etc.
• Voluntary motion can be controlled by keeping
  examination time short and asking the patient to
  remain still during the examination

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Lack of sharpness due to motion (2)

• Shorter exposure times are achieved by the use of
  fast intensifying screens
• N.B. Faster screens result in loss of details
  (receptor sharpness)
• Further, the use of shorter exposure time has to
  be compensated with increased mA to achieve a
  good image
• This often implies use of large focal spot
  (geometric sharpness)

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Lack of receptor sharpness

• The intensifying screen in radiography has a
  crystal size which is larger than that of the
  emulsion on the film
• An image obtained without the screen will be
  sharper than that obtained with the screen, but
  will require much more dose
• The thickness of the screen further results in
  degradation of sharpness
• On digital imaging, the image displayed at a
  higher matrix with a finer pixel size has better
  clarity

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Part 8 Image quality
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 4 Distortion and artifacts

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Distortion and artifacts

• Unequal magnification of various anatomical
  structures
• Inability to give an accurate impression of the
  real size, shape and relative positions
• Grid artifact (grid visualized on the film)
• Light spot simulating microcalcifications (dust
  on the screen)
• Bad film screen contact, bad patient positioning
  (breast)

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Distortion and artifacts
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Distortion and artifacts
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Part 8 Image quality
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 5 Image noise

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Noise

• Defined as uncertainty or imprecision of the
  recording of a signal
• Impressionist painting precision of object
  increases with number of dots
• X Ray imaging when recorded with small number of
  X- photons has high degree of uncertainty,more
  photons give less noise
• Other sources of noise
• Grains in radiographic film
• Large grains in intensifying screens
• Electronic noise of detector or amplifier

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Noise in film

• Noise is characterized by the standard deviation
  (s) of the OD measurements in any uniform region
  of the film

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Image noise

• Information that is not useful is noise
• The snowing in a TV image, the speckles in an
  ultrasound image are examples of noise
• Noise interferes with visualization of image
  features useful for diagnosis
• Different components of noise are
• Radiation noise (heel effect)
• Structure noise (Compton scattering)
• Receptor noise (non-uniform response to a uniform
  X Ray beam)
• Quantum mottle (low photon flux)

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Summary

• Different technical and physical factors may
  influence the image quality by impairing the
  detection capability of the anatomical structures
  useful for diagnosis (increasing the image
  unsharpness)
• Some factors depend on the receptor, some others
  are more related to the radiographic technique

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Where to Get More Information

• Hendee WR, Riternour ER, eds. Medical Imaging
  physics, 3rd ed. St. Louis Mosby Year Book, 1992
• Sprawls Perry Jr. Ed. Physical principles of
  medical imaging. Maddison Medical Physics
  Publishing, 1993
• Moores BM, Wall BF, Eriskat H and Schibilla H,
  eds. Optimization of image quality and patient
  exposures in diagnostic radiology. London
  British Institute of Radiology 1989.